how to sign an already compiled APK

I've decoded an APK with apktool (as the original source code was lost) so I could fix some issues with the layout xml files. I've then rebuilt it back up with apktool and when I tried to install it on my device (using adb: adb install appname.apk) it gave me this error:

[INSTALL_PARSE_FAILED_NO_CERTIFICATES]

the original apk however was signed by a keystore (on eclipse IDE), this one isn't, how can I sign it properly with it's original keystone file outside Eclipse!?